1. Field of the Invention
The invention relates generally to data encoding systems and, in particular, to systems that utilize running digital-sum encoding.
2. Background Information
Before recording or transmission, data are typically encoded for error protection and also for modulation, to preserve signal content against corruption by noise, fading and other interference associated with a communications channel in the recording or transmission system. The modulation encoding is also performed to preserve signal content that might otherwise be filtered out by circuitry in the communications channel. For example, a preamplifier in the circuitry acts essentially as a high pass filter, and filters out much of the low frequency content of the information signal. In certain recording systems, such as, for example, perpendicular recording systems, there is significant energy in the low frequency portion of the information signal. Accordingly, the filtering out of the low frequency content would result in performance degradation. The data are thus encoded to preserve the low frequency content. One example of encoding to preserve the low frequency content is running-digital-sum (“RDS”) encoding, which constrains the direct-current (“DC”) content of the signal.
Typically, the user data are encoded to produce RDS encoded data and the RDS encoded data are further encoded using an error correction code to produce error correction code words. The error correction code words are then RDS encoded and the result is further encoded using an outer block code, such as an iterative parity check code, to protect against errors in reproducing the error correction code words at the receive end of the channel. The parity check encoding is not typically followed by RDS encoding because the RDS encoding complicates the iterative block decoding, requiring mapping to take place during the iterative steps in which updated soft information is passed between a channel detector and the decoder.
Before entering the communications channel, the encoded data and the parity check bits are provided to a precoder, which translates them into bit sequences that are optimized for transmission and detection. The preceding, however, does not preserve the RDS constraints, and there is thus a trade-off between the optimization for transmission/detection and the encoding to preserve signal content through the channel circuitry.